Unlocked AT&T Galaxy S4 not working properly

My fried bought a Galaxy S4 from AT&T when he was in US. While leaving the country he bought an unlock code from AT&T and unlocked the phone using the same in India. His phone was working fine initially. Then sudden signal loss started to occur and need to restart the phone to get the signal back. Tried using different Indian carriers and the same issue continued. Will I be able to solve this issue by flashing a custom rom into his phone? Do I need to take care of anything else, like the modem firmware or something. Attaching a screen shot of his phone details. Please help guys..

My fried bought a Galaxy S4 from AT&T when he was in US. While leaving the country he bought an unlock code from AT&T and unlocked the phone using the same in India. His phone was working fine initially. Then sudden signal loss started to occur and need to restart the phone to get the signal back. Tried using different Indian carriers and the same issue continued. Will I be able to solve this issue by flashing a custom rom into his phone? Do I need to take care of anything else, like the modem firmware or something. Attaching a screen shot of his phone details. Please help guys..

Sent from my Nexus 4 using XDA Premium 4 mobile app

You could try reflashing the same MDL firmware first. If that doesn't work, then install TWRP and try the different flashable modems.

Thank you for the reply.. However we have already tried a factory reset.. Will it be a problem if I flash the modem firmware available in the Indian Galaxy S4?? The Indian version comes with an exynos octa processor instead of the Qualcomm one..

Thank you for the reply.. However we have already tried a factory reset.. Will it be a problem if I flash the modem firmware available in the Indian Galaxy S4?? The Indian version comes with an exynos octa processor instead of the Qualcomm one..

Sent from my Nexus 4 using XDA Premium 4 mobile app

Doing a factory reset is different from reflashing the firmware. I don't think the other modems would work, but I'm not entirely sure. If they're recovery zips you could try, but if they're ODIN files it definitely won't work.

I have tried reflashing the phone with Odin. The device now has the latest Android 4.4.2 firmware from AT&T. It seemed to work fine after flashing. We got the network and were able to make calls as well. However after sometime the signal got cutoff itself. Tried flashing again, the same thing happens. Any other suggestions??

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